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others name them fibrils, - a term more generally adopted ; while the terms rhizina and rhizula have been given by Link to the young roots of mosses and lichens.
A fibril is a little bundle of bothrenchyma, or sometimes of trachenchyma, encased in woody fibre, and covered by a lax cellular integument: it is in direct communication with the vascular system of the root, of which it is, in fact, only a subdivision ; and its apex consists of extremely lax cellular tissue and mucus. This apex has the property of absorbing fluid with great force, and has been called by De Candolle the Spongiole or Spongelet. It must not be considered a particular organ; it is only the newly formed and forming tender tissue. In Pandanus the spongelets of the aërial roots consist of numerous very thin exfoliations of the epiphlæum, which form a sort of cup fit for holding water in.
The proportion borne by the root to the branches is extremely variable: in some plants it is nearly equal to them, in others, as in Lucerne, the roots are many times larger and longer than the stems; in all succulent plants and in Cucurbitaceæ they are much smaller. When the root is divided into a multitude of branches and fibres, it is called fibrous : if the fibres have occasionally dilatations at short intervals, they are called nodulose. When the main root perishes at the extremity, it receives the name of præmorse, or bitten off: frequently it consists of one fleshy elongated centre tapering to the extremity, when it is termed fusiform (or tap-rooted by the English, and pivotante by the French)* : if it is terminated by several distinct buds, as in some herbaceous plants, it is called many-headed (multiceps).
The roots of many plants are often fleshy, and composed of lobes, which appear to serve as reservoirs of nutriment to the
* In the former editions of this work the turnip has been referred to a root. But, from the investigations of Turpin and others, there is no room to doubt that the turnip, the radish, the cyclamen, and the elephant-foot, are all distensions of the stem : either of the first internodium, or of the inferior prolongation of the stem below the coʻyledons and above the root.
fibrils that accompany them; as in many 46 terrestrial Orchidaceous plants, Dahlias, &c. These must not be confounded either with tubers or bulbs, as they have been by some writers, but are rather to be considered a special form of the root, to which the name of Tubercules (fig. 46.) would not be inapplicable. In Orchis the tubercules are often palmated or lobed; in the Dahlia, and many Asphodeleæ, they hang in clusters, or are fasciculated.
In internal structure the root differs little from the stem, except in being often extremely fleshy ; its cellular system being subject to an unusually high degree of developement in a great many plants, as the Parsnip, and other edible roots. In Endogens, the mutual arrangement of the cellular and vascular systems of the root and stem is absolutely the same; but in Exogens, there is never any trace of pith in the root.
Sect. IV. Of the Appendages of the Axis.
From the outside of the stem, but connected immediately with its vascular system, arises a variety of thin flat expansions, arranged with great symmetry, and usually falling off after having existed for a few months. These are called, collectively, appendages of the axis; and, individually, scales, leaves, bracts, flowers, sexes, and fruit. They must not be confounded with mere expansions of the epidermis, such as ramenta, already described (p. 63.), from which they are known by having a connection with the vascular system of the axis. Till lately, botanists were accustomed to consider all these as essentially distinct organs; but, since the appearance of an admirable treatise by Goethe in 1790, On the Metamorphoses of Plants, proofs of their being merely modifications of one common type, the leaf, have been gradually discovered; so that that which, forty years ago, was considered as the romance of a poet, is now universally acknowledged to be an indisputable truth. It may, however, be remarked, that when those who first seized upon the important but neglected facts out of which this theory has been constructed, asserted that all appendages of the axis of a plant are metamorphosed leaves, more was stated than the evidence at that time would justify; for we cannot say that an organ is a metamorphosed leaf, when, in point of fact, it has never been a leaf. What was meant, and that which is supported by the most conclusive evidence, is, that every appendage of the axis is originally constructed of the same elements, arranged upon a common plan, and varying in their manner of developement, not on account of any original difference in structure, but on account of especial, local, and predisposing causes : of this the leaf is taken as the type, because it is the organ which is most usually the result of the developement of those elements, is that to which the other organs generally revert, when, from any accidental disturbing cause, they do not sustain the appearance to which they were originally predisposed, - and moreover, is that in which we have the most complete type of organisation.
It is not my intention just now to enter into separate discussion of this doctrine; proof of it will be more conveniently adduced as the different modifications of the
appendages of the axis come respectively under consideration. The leaf, as the first that is formed, the most perfect of them all, and that which is most constantly present, is properly considered the type from which all the others are deviations, and is the part with the structure of which it is first necessary to become acquainted.
51 The leaf is an expansion of the bark at the base of a leafbud, prior to which it is developed. In most plants it consists of cellular tissue, filling up the interstices of a net-work of fibres which proceed from the stem, and ultimately separating from the bark by an articulation; in many Monocotyledonous plants, Ferns and Mosses, no articulation exists, and the base of the leaf only separates from its parent stem by rotting away.
This difference of organisation has given rise to a distinction, on the part of Oken, between the articulated leaves of Dicotyledons and the inarticulated leaves of Monocotyledons and Acotyledons: the former he calls true leaves, and distinguishes by the name of Laub; the latter he considers foliaceous dilatations of the stem, analogous to leaves, and calls Blatt.
A leaf consists of two parts; namely, its stalk, which is called the petiole (fig. 48. a), and its expanded surface, which is called the blade or lamina (fig. 48. c, b, d): in ordinary language the latter term is not employed, but in very precise descriptions it is indispensable.
The point where the base of the upper side of a leaf joins the stem is called the axil; any thing which arises out of that point is said to be axillary. If a branch or other process proceeds from above the axil, it is called supra-axillary; if from below it, infra-axillary.
The scar formed by the separation of a leaf from its stem is sometimes called the cicatricule. The withered remains of leaves, which, not being articulated with the stem, cannot fall off, but decay upon it, have been called reliquiæ or induvia, and the part so covered is said to be induviate.
When leaves are placed in pairs on opposite sides of a stem (fig. 53.), and on the same plane, they are called opposite : if more than two are opposite, they then form what is called a whorl, or verticillus : but if they arise at regular distances from each other round the stem, and not from the same plane, they are then called alternate.
In plants having Exogenous stems, the first leaves, namely, those which are present in the embryo itself (cotyledons),
are uniformly opposite ; but those subsequently developed are either opposite verticillate, or alternate in different species : on the contrary, in Endogens, the embryo leaf is either solitary, or, if there are two, they are alternate; and those subsequently developed are usually alternate also, but few cases occurring in which they are opposite. Hence some have formed an opinion that the normal position of the leaves of Exogens is opposite, or verticillate; and that when the leaves are alternate, this arises from the extension of a node; while that of Endogens is alternate, the whorls being the result of the contraction of internodes.
But it seems more probable that the normal position of all leaves is alternate, and their position upon the stem an elongated spiral, as is in many cases exceedingly apparent, as, for instance, in the genus Pinus, in Pandanus, which is actually named Screw-pine, in consequence of the resemblance its shoots bear to a screw, and in the Pine-apple: the Apple, the Pear, the Willow, the Oak, will also be found to indicate the same arrangement, which is only less apparent because of the distance between the leaves, and the irregularity of their direction. If, in the Apple-tree for instance, a line be drawn from the base of one leaf to the base of another,